Integrated circuits formed using complementary-metal-oxide-semiconductor (CMOS) processes for dynamic random access memory (DRAM) applications typically use memory cells consisting of a single transistor and capacitor to minimize area. Stored charge on the capacitor representing a data state is accessed when a selected wordline turns on the associated transistor. This results in charge sharing between the cell capacitor and bitline connected to source and drain regions of the transistor. Because the capacitance of the bitline is much larger than that of the memory cell, the change in voltage after charge sharing may be quite small and is given by the equation Vf=(Vbl+A*Vc)/(1+A) where Vbl is the bitline precharge voltage, Vc is the memory cell voltage and A is the ratio of storage cell capacitance to bitline capacitance.
Practical operation of the memory device is further constrained by the fact that stored charge in the memory cell gradually leaks away in an asymmetrical manner. For example, a memory cell with a voltage representing a logical one might leak much more than a memory cell with a voltage representing a logical zero. In order to overcome these limitations typical DRAMs utilize complementary bitline pairs for differential sensing of the small signal. Additionally, a special cell or dummy cell is often connected to the complementary bitline and precharged to a reference potential. After charge sharing the complementary bitline voltage is given by Vf'=(Vbl+B*Vd)/(1+B) where Vbl is the bitline precharge voltage, Vd is the dummy cell voltage and B is the ratio of dummy cell capacitance to bitline capacitance. Adjustment of either the dummy cell size or voltage will modify Vf' so that the signal available for sensing (Vf-Vf') is approximately the same even after asymmetrical cell leakage.
In practice, each bitline of the complementary pair has an equal number of memory cells. Previous designs have typically used at least two and perhaps four dummy cells for each sense amplifier so that selection of a memory cell on one bitline corresponds to selection of a dummy cell on the complementary bitline. Since a typical 16 megabit DRAM may have more than 34,816 sense amplifiers, a considerable amount of area is expended on dummy cells. The present invention describes a means to utilize only a single dummy cell for each sense amplifier thereby reducing required area and circuit complexity.